1. Field of the Invention
The present invention relates to a method for forming a field oxide of a semiconductor device and to the semiconductor device. More particularly, the present invention relates to a novel LOCOS (LOCal Oxidation of Silicon) technique by which semiconductor devices can be highly integrated.
2. Description of the Prior Art
In order to better understand the background of the invention, a description will be given of conventional techniques in conjunction with some figures.
Referring to FIG. 1, there is a conventional method of forming an element isolation film of a semiconductor device.
First, there is prepared a semiconductor substrate 11 over which a pad oxide 12 and a nitride 13 are sequentially formed, as shown in FIG. 1a. The nitride 13 and the pad oxide 12 are selectively etched at a field region to provide an element isolation mask which is, then, entirely covered with a nitride. It is dry-etched to form a nitride spacer at the side wall of the element isolation mask. After this dry-etching, a nitrogen-containing polymer P always remains on the field region of the semiconductor substrate.
FIG. 1b is a cross section after the exposed region of the semiconductor substrate 11 is oxidized at 800-900.degree. C. to grow a recess-oxide 15. During this oxidation, the polymer P diffuses into the semiconductor substrate 11 below the recess oxide 15.
FIG. 1c is a cross section after the recess oxide 15 is subjected to wet-etching to form a trench T in the semiconductor substrate 11, followed by the oxidation of the exposed semiconductor substrate 11 to form a field oxide 16.
As mentioned, the conventional technique has an advantage of forming a trench at an accurate depth in a semiconductor substrate by wet-etching the recess oxide but is disadvantageous in that the nitrogen-containing polymer remains on the field region of the semiconductor substrate as a result of the dry-etchings.
Whereas the formation of the nitrogen-containing polymer occurs minorly in the place where the area ratio of active region to field region is small, such as a cell region, a large amount of the nitrogen-containing polymer is formed in peripheral circuits in which the area of active region is relatively very great as compared with that of field region.
The nitrogen-containing polymer which is formed during the nitride etching steps, is released outside or redeposited on gully-shaped field regions, but because the polymer is formed at a large amount in peripheral circuit regions, a part of the polymer remains on the bottom of the field region.
The nitrogen-containing polymer is not easily removed by conventional wet washing as it binds to the semiconductor substrate.
Because the recess-oxide is very thin, the oxidation is generally carried out at a low temperature of approximately 800-900.degree. C. to easily control the thickness of the oxide. In this case, the nitrogen in the polymer P shown in FIG. 1a is redistributed in the interface between the recess oxide and the semiconductor substrate upon the growth of the recess oxide 15 and the redistributed nitrogen-containing polymer still remains in the semiconductor substrate even after the removal of the recess oxide with a hydrofluoric acid etchant, as shown in FIG. 1c.
In general, the element isolation processes based on LOCOS yield a field oxide thinning effect, in which thinner field oxides are grown as the oxidation window is narrower and the field oxidation temperature is lower. The data of FIG. 2 show that the field oxide thinning effect can be overcome by increasing the field oxidation temperatures. If a serious field oxide thinning effect occurs, threshold voltages of parasitic transistors and punchthrough voltages are disadvantageously lowered.
However, when the temperature at which the field oxide is grown is as high as or higher than 1,050.degree. C., the nitrogen-containing polymer of FIG. 1c is thermally activated and instantaneously transformed into a nitride which acts as a barrier against the growth of the field oxide. In result, a field-oxide-ungrowth (hereinafter referred to as "FOU") phenomenon occurs. FIG. 1d shows such an FOU phenomenon in which the field oxide is not grown at its center. Particularly, if the FOU phenomenon occurs at even one place of the peripheral circuits, the field oxide is not formed, so that an electrical short occurs, making it impossible for the devices to function.
Therefore, the conventional techniques which take advantage of high temperatures of 1,050.degree. C. or higher to improve the field oxide thinning effect, have an inevitable problem to be solved, the FOU phenomenon.